EUCALYPT HYBRIDS

A few years ago I produced a cross between a young River Red Gum tree growing on our place and some large mature Yellow Box trees.
This happened after taking seed from the vigorous young Red Gum and finding that the resultant seedlings were about 50/50 pure River Red Gum and some unknown box-like seedling.

I took the seedlings into CSIRO (Commonwealth Scientific Industrial Research Organisation) where a eucalypt expert, Mr Ian Bruckner, confirmed that they were hybrids, (River Red Gums hybridise readily and there is no reason why they would not cross with E.Melliodora – Yellow Box). Mr Bruckner said that though he was 99% sure of its hybrid nature, he would nevertheless like to further confirm after seeing the flowers of the suspected hybrids.
I am still waiting for them to flower, but their growth and interesting behaviour seems to further confirm their E.MelliodoraXE.Camaldulensis hybrid nature. Before I discuss their present growth habits, I will digress and discuss why the two trees are of particular interest.

Ecological considerations

When I saw representatives of CSIRO back in 1994, they told me that their work in hybridising eucalypts mostly revolved around the softer pulpwood species. They were concentrating on crossing species so that pulpwood plantation farmers could grow pulpwood trees in colder areas. Admirable as this might be, the slower growing Ironbarks and Boxwood trees are not regenerating in their native habitat, and their timber, which is the hardest and heaviest in the world, and which cannot be excelled for its durability and myriad uses, from heavy construction to railway sleepers and fence posts, is becoming more scarce, and is being replaced by shoddier products.
(Ironbark is so heavy, the early settlers found that they could not build boats out of it, as it would sink.)
The CSIRO magazine ECOS said in 1994 about natural Boxwood woodlands in Australia:

Before European settlement, grassy box woodlands covered millions of hectares between Southern Queensland and Northern Victoria. They featured eucalypt species such as Yellow Box, Grey Box and White Box with an understorey of kangaroo grass, wallaby grass, and snowgrass and wildflowers such as yam daisies, donkey orchids and chocolate lilies.
Scattered trees remain, but native understorey has been eliminated from most sites. Prober and her colleagues are studying the ecology and genetics of remnant grassy White Box woodlands to develop a strategy for conserving threatened woodland ecosystems. They estimate that less than 50 hectares of the original woodlands remain intact.

The Box species are disappearing and regeneration is not happening. Originally growing in the more fertile valleys and woodlands rather than ridges, they were in the way of what looked like good grazing land, and were zealously cleared.
Regrowth is subject to insect attack which kills many trees, and farmers, who prize Boxwood for its uses as fenceposts and as a fiercely hot and slow burning fuel, lose patience when it comes to waiting for these trees to regrow. The average growth rate of Yellow Box is about one inch a year. I have some naturally regenerating Yellow Box trees on my place which in ten years don’t appear to have grown at all, and some have even been retarded in their growth by the native pests that plague them. At their young stages of growth many regenerating Box trees are lost to pests.
They become too big and dangerous to be cultivated as park or backyard trees. Like most eucalypts, they eventually drop branches, and these outlandishly heavy branches are often the size of a medium tree. They are definitely farm trees.
In Australia, you still pay extra for firewood that can be guaranteed to be Yellow Box, and its very desirability as a fuel has helped in the tree’s demise.
E. Camaldulensis, (River Red Gum) on the other hand is still the most widespread eucalypt species in Australia and exists all over Australia, not just in that eastern belt that characterises the box woodlands. Being more straight grained than E.Melliodora it has been harvested widely for construction purposes, and originally contributed heavily to Australia’s railway sleepers in the construction of railways. Along the Murray river where it was most widespread, astronomical amounts of mature trees have disappeared, being cleared in the wake of the food and irrigation belt, and now more are many dying because of altered flood plains and salinity. Unlike Yellow Box which is a tree of the high dry fertile plains, River Red Gum is a tree of rivers and flood plains. Its heavy red timber is ideally suited to heavy construction.

Both trees are heavy honey producers, and Yellow Box honey is one of the finest honeys available anywhere.

Growth habits

I have already mentioned the slow growth rate which has contributed to Yellow Box’s inability to compete in regeneration – particularly when this is combined with the introduction of choking exotic plant species and the tendency to be prone to devastating insect attacks which set it back even further when they don’t kill it. Once upon a time the insects would have killed off a certain quota, leaving others to continue to grow. Now there are fewer yellow box trees regenerating and the pests have less to choose from. More are prone to various degrees of blight.
River Red Gum on the other hand is a relatively robust tree and a relatively fast grower. It is more sensitive to fire than Yellow Box (it has no lignotuber and does not regenerate easily after a fierce bushfire) but all in all, is a more disease resistant tree. It is also more frost prone than the frost resistant box trees, but again, it shows great variation within the species, and River Red Gums from some areas (such as Victoria for instance) are more cold resistant than River Red Gums growing elsewhere (such as the Northern Territory).
I was experimenting growing River Red Gum in an extremely cold area (near Canberra, which is not it’s native habitat) and these particular trees which came from Victoria proved remarkably successful. One grew 7 metres tall and 5 metres wide (it had a remarkably leafy and wide crown) in 10 years. For all its setbacks suffered in an inordinately cold climate, it grew remarkably quickly. It was from this tree I took seed and discovered it had hybridised with the locally native (but dying) Yellow Box trees.

Why hybridise hardwoods?

It has been the Australian hardwoods that were most taken for granted and seen as an infinite resource, which now is no more. They were burnt and still are, widely, with their slow, hot burning timber being prized for fuel. The timber, being hard to work, was substituted for Radiata Pine in the building industry, but nothing can replace hardwood’s durability and termite resistance. Softer timber eucalypts are sought after for the pulpwood and paper industry, and heavy timber railway sleepers have been replaced by concrete ones. We still however, chose to build our house out of Australian hardwoods (even recycled bridge timbers) and though the construction was slower and required heavy power tools, the result is one that is more durable and pleasing. When dressed and polished, the dense grained hardwoods are excellent for funiture making, exposed building timbers and other purposes. I would like to believe that the heavy hardwoods are there for future generations. In inadvertently crossing Yellow Box and River Red Gum , I hopefully also have some good honey producing hybrids growing. Time will tell.

Results so far

The hybrids I have growing on our land are growing in the most adverse conditions possible, suffering about 5 months of frosts a year, and bitterly cold winds. I am sure they would be growing faster in more optimum conditions. Nine were planted in 1994 and all nine are alive and thriving. I intend to generate some more. So far, they have displayed what is called hybrid vigour. They look like a disease resistant version of a young Yellow Box (see photo below) but grow at a rate comparable to a River Red Gum. Notice the photo is of a young hybrid growing next to a control River Red Gum and they are of comparable growth rates, about five feet in six years, which I admit is not fast, but which I’m sure would be increased in a more hospitable climate. Both require relatively fertile, or at least clayey soils. I expect that like its parents, the hybrid will not be a backyard tree, as both trees are large, growing to at least thirty metres eventually.
I am particularly interested in keeping the Yellow Box genes alive and would hate to see the demise of this beautiful and useful tree.